Processes for preparing synthesis gas using natural gas may be classified generally into steam reforming of methane (SRM), partial oxidation of methane (POM), and carbon dioxide reforming of methane (CDR). In each reforming process, the ratio of hydrogen to carbon monoxide (H2/CO) may be varied with the optimal condition required for the subsequent process. For example, in the case of a highly endothermic SRM process, it is possible to obtain a ratio of H2/CO of 3 or higher. Thus, the process is suitable for hydrogen production and ammonia preparation. In the case of POM process, a ratio of H2/CO of about 2 is obtained. Thus, the process is suitable for methanol synthesis and hydrocarbon formation through Fischer-Tropsch synthesis. Hereinafter, the above-mentioned reforming processes are outlined with their advantages, disadvantages and the values of heat of reactions.
Steam Reforming of Methane (SRM)CH4+H2O=3H2+CO ΔH=226 kJ/mol
→high endothermic reaction, H2/CO>3, excess steam is required.
Partial Oxidation of Methane (POM)CH4+0.5O2=2H2+CO ΔH=−44 kJ/mol
→mild exothermic reaction, H2/CO=2, O2 production process is required.
Carbon Dioxide Reforming of Methane (CDR)CH4+CO2=2H2+2CO ΔH=261 kJ/mol
→high endothermic reaction, H2/CO=1, CO2 addition is required.
In addition to the above reforming processes, there are known an auto-thermal reforming (ATR) process including POM combined with SRM, a tri-reforming process including POM combined with SRM and CDR, or the like in order to maintain an adequate H2/CO ratio as well as to increase energy and carbon efficiency. Further, it is possible to obtain synthesis gas having different H2/CO ratios depending on the type of reforming process and catalyst. Recently, many patent applications related to different methods using synthesis gas having such different ratios have been made (Korean Patent Publication Nos. 2006-0132293 and 2005-0051820).
The present disclosure relates to a nickel-based catalyst for combined reforming for preparing synthesis gas, wherein the combined reforming is carried out by SRM combined with CDR, and the synthesis gas is suitable for methanol synthesis and Fischer-Tropsch synthesis. In general, it is known that a ratio of H2/(2CO+3CO2) in synthesis gas of about 1.05 is thermodynamically suitable for methanol synthesis. As the ratio increases up to 1.05, methanol yield also increases. Therefore, it is required to add hydrogen or to adjust the conversion in CDR in order to adjust the above ratio. In addition, in the case of Fischer-Tropsch synthesis using an iron-based catalyst, surplus hydrogen is produced during the reaction due to high activity of water gas shift (CO+H2O=H2+CO2) reaction and high selectivity toward CO2 is provided. Such iron-based catalysts have their unique high activity, and allow progress of Fischer-Tropsch synthesis even in the presence of a low molar ratio of synthesis gas (H2/CO=0.5-0.7) containing CO2, due to high conversion of water gas.
In the case of a currently available SRM process, a Ni/Al2O3 catalyst system is used at a reaction temperature of 750-850° C. under a molar ratio of steam/methane of 4-6:1. However, such a catalyst system at low H2O/CH4 ratio is problematic in that it undergoes severe deactivation caused by carbon deposition. Therefore, to solve that problem, many studies have been conducted about catalyst systems containing noble metals or transition metals and alkali metals as co-catalysts (Journal of Molecular Catalysis A 147 (1999) 41).
In the case of a CDR process, more severe deactivation of catalysts occurs due to carbon deposition. Therefore, in order to inhibit such catalyst deactivation, many studies have been conducted about noble metal catalysts (Pt/ZrO2) and Ni/MgO or Ni/MgAlOx catalyst systems, to which alkali metals are added as co-catalysts (Catalysis Today 46 (1998) 203, Catalysis communications 2 (2001) 49, and Korean Patent Publication No. 10-2007-0043201). In general, when using commercially available SRM catalysts directly to CDR process and combined CDR and SRM processes, deactivation of catalysts caused by carbon deposition is accelerated. According to our previous studies (Korean Patent Publication Nos. 2002-0021721, 2004-0051953 and 2002-0088213), several methods for inhibiting catalyst deactivation caused by carbon deposition are disclosed, wherein catalysts are prepared by supporting nickel on a zirconia carrier or alumina carrier modified with cerium, or by carrying out co-precipitation of cerium, zirconium and nickel.
According to the present disclosure, nickel used as a catalytically active ingredient is supported on a MgAlOx metal oxide carrier pretreated with either or both of cerium and zirconium, so that the resultant catalyst is used for combined reforming (SRM+CDR) of methane to produce synthesis gas having a composition suitable for methanol synthesis and Fischer-Tropsch synthesis, while inhibiting deactivation of the catalyst caused by carbon deposition.
Although many workers have participated in studies for improving the quality of methanol synthesis catalysts, complete understanding about the active site of a catalyst for methanol synthesis cannot be accomplished. However, it is known that oxidation state of Cu and redox property of reduced Cu particles play an important role in determining the catalyst quality. It is also known that the activity of a Cu catalyst in a reaction of methanol synthesis is in proportion to the specific surface area of Cu of the metal components. It is reported that coordination, chemisorption, activation of CO and homogeneous H2 splitting occur on Cu0 or Cu+, and non-homogeneous H2 splitting, leading to Hδ+ and Hδ− in a catalytic process using a ZnO-containing catalyst, occurs on ZnO (Appl. Catal. A 25, (1986) 101). Herein, it is reported that when the molar ratio of Cu/Zn is 8 or more, the specific surface area decreases rapidly (Appl. Catal. A 139, (1996) 75). For this reason, Cu is used in combination with Zn to prepare the catalyst, and a molar ratio of Cu/Zn of 3/7 is known to provide the highest activity. However, it is known that when CO2 is present or when the proportion of oxygen-containing materials that cover the Cu0 surface increases, the catalyst activity is independent from the Cu0 surface area. It is also reported that such a phenomenon results from the fact that the Cu+ active site functions as an active site during the methanol synthesis.
Meanwhile, Fischer-Tropsch synthesis provides a product/byproduct distribution that varies with the catalytically active ingredient used therein. In general, iron-based catalysts and cobalt-based catalysts are used. The Fischer-Tropsch processes using iron-based catalysts produce a surplus amount of hydrogen and provide high selectivity toward CO2 due to high conversion of water gas (CO+H2O=H2O+CO2). However, such iron-based catalysts used in Fischer-Tropsch synthesis are cheaper than cobalt, have specifically high activity, and allow progress of Fischer-Tropsch synthesis even under a low molar ratio of synthesis gas (H2/CO=0.5-0.7) containing CO2 due to high activity of water gas shift reaction. Therefore, due to high activity toward water gas conversion, such Fischer-Tropsch processes using iron-based catalysts are advantageous in treating synthesis gas having a low H2/CO ratio. On the other hand, since cobalt-based catalysts have low activity toward water gas conversion, they perform Fischer-Tropsch synthesis using synthesis gas having a high molar ratio of hydrogen/carbon monoxide (H2/CO=1.6-2.2) produced from natural gas. The iron-based catalysts for use in Fischer-Tropsch synthesis may be obtained by a melting or precipitation method, as well as by a spray drying method. It is also reported that the iron-based catalysts obtained by spray drying have improved wear resistance and improved physical strength while maintaining catalytic activity (Industrial & Engineering Chemistry Research 40 (2001) 1065). In addition, iron-based catalysts generally include at least one co-catalyst that helps absorption of CO or reduction of iron. Particularly, addition of potassium to a precipitated iron catalyst increases the yield of a high-molecular weight product and improves catalytic activity. Besides potassium, copper may be used as a co-catalyst for Fischer-Tropsch synthesis using iron-based catalysts in order to accelerate reduction of iron. Copper accelerates reduction of iron and is more effective in terms of the reaction rate in Fischer-Tropsch synthesis. However, since copper reduces the activity toward water gas conversion, it is not possible to maintain a ratio of H2/CO suitable for Fischer-Tropsch synthesis. To overcome this, C5+ hydrocarbons may be prepared selectively under a high CO conversion by using, as co-catalysts, a cooper and a 1A or 2A metal element with iron-manganese using no carrier (U.S. Pat. No. 5,118,715). In the preparation of an iron catalyst, it is advantageous that the catalyst has a large specific surface area in order to disperse small metal particles and to stabilize the catalyst. Thus, a binder as a structural stabilizer may be added in combination with a co-catalyst to an iron catalyst system.
Therefore, the present disclosure provides a method for preparing synthesis gas by carrying out a combined reforming (SRM+CDR) process using a nickel-based reforming catalyst (Ni/Ce(Zr)/MgAlOx) having high catalytic activity, so that the resultant synthesis gas maintains an adequate composition of carbon monoxide, carbon dioxide and hydrogen [H2/(2CO+3CO2)], and is useful for methanol synthesis and Fischer-Tropsch synthesis using an iron-based catalyst.